Add-on is the Mozilla term for software modules that can be added to the Firefox web browser and related applications. Mozilla hosts them on its official add-on website.

Browser extension A browser extension is a small software module for customizing a web browser. Browsers typically allow a variety of extensions, including user interface modifications, cookie management, ad blocking, and the custom scripting and styling of web p ...

s are the primary type of add-on. In 2017, Mozilla enacted major changes to the

application programming interface An application programming interface (API) is a way for two or more computer programs to communicate with each other. It is a type of software interface, offering a service to other pieces of software. A document or standard that describes how t ...

(API) for extensions in Firefox, replacing the long-standing XUL and

XPCOM Cross Platform Component Object Model (XPCOM) is a cross-platform component model from Mozilla. It is similar to Microsoft Component Object Model (COM) and Common Object Request Broker Architecture (CORBA). It features multiple language bindings an ...

APIs with the

WebExtensions API Mozilla Firefox has features that allow it to be distinguished from other web browsers, such as Chrome and Internet Explorer. Major differences To avoid interface bloat, ship a relatively smaller core customizable to meet individual users' ne ...

that is modeled after

Google Chrome Google Chrome is a cross-platform web browser developed by Google. It was first released in 2008 for Microsoft Windows, built with free software components from Apple WebKit and Mozilla Firefox. Versions were later released for Linux, macOS ...

's API. Thus add-ons that remain compatible with Firefox are now largely compatible with Chrome as well.

Current add-ons

Extensions

WebExtensions

Starting with Firefox 57, only the new

is supported.

Themes

Early versions of Firefox supported themes that could greatly change the appearance of the browser, but this was scaled back over time. Current themes are limited to changing the background and text color of toolbars. (These lightweight themes were formerly called personas.)

Historical add-ons

Extensions

Legacy extensions

Prior to 2017, Firefox supported extensions developed with different APIs: XUL,

, and

Jetpack A jet pack, rocket belt, or rocket pack is a device worn on the back which uses jets of gas or liquid to propel the wearer through the air. The concept has been present in science fiction for almost a century and became widespread in the 1960s. ...

. Mozilla now refers to these as legacy extensions.

Plug-ins

Plug-ins are no longer supported in Firefox. In the past, they were used to handle

media type A media type (also known as a MIME type) is a two-part identifier for file formats and format contents transmitted on the Internet. The Internet Assigned Numbers Authority (IANA) is the official authority for the standardization and publication o ...

s for which the application did not have built-in capability. They were deprecated due to security concerns and improvements in Web APIs. The last one that was officially supported was Adobe Flash Player, which Adobe discontinued in 2020.

Restrictions

Mozilla had no mechanism to restrict the privileges of legacy Firefox extensions. This meant that a legacy extension could read or modify the data used by another extension or any file accessible to the user running Mozilla applications. But the current WebExtensions API imposes many restrictions. Starting with Firefox 40, Mozilla began to roll out a requirement for extension signing. It is now required in all official Firefox releases.

Website

The Mozilla add-ons website is the official repository for Firefox add-ons. In contrast to

mozdev.org mozdev.org was a website that offered free project hosting, and software development tools to the Mozilla community. Site hosted extensions for Firefox, Thunderbird and SeaMonkey and stand-alone Mozilla-based applications. It was free to set up a ...

which provides free hosting for Mozilla-related projects, the add-ons site is tailored for users. By default, Firefox automatically checks the site for updates to installed add-ons. In January 2008, Mozilla announced that the site had accumulated a total of 600 million add-on downloads and that over 100 million installed add-ons automatically check the site for updates every day. In July 2012, the total had increased to 3 billion downloads from the site.

References

External links

Official add-on website

WebExtensions API reference documentation

Extension Workshop, Mozilla's site for Firefox extension developer documentation
{{Mozilla Mozilla Free software websites